Errors in time-difference-of-arrival (TDOA) and frequency-difference-of-arrival (FDOA) measurements used to perform geolocation of non-cooperative emitters (i.e., signals of unknown format or content, transmitted from an unknown location) can be reduced by use of a reference beacon that transmits a signal from a known position. By performing a reverse geolocation using signals transmitted from a reference beacon having a known position and velocity, it is possible to estimate bias errors in the TDOA/FDOA measurements taken by the geolocation system. The estimated bias errors can then be subtracted from measurements taken of non-cooperative emitter signals to reduce the bias error in those measurements. Such correction is typically performed by a reference correction processor that is part of or operates in conjunction with the geolocation system.
Typically, TDOA/FDOA collection receivers do not, or cannot, differentiate between signals received from a non-cooperative (or target) emitter and signals received from a reference beacon. It is up to the geolocation or reference correction processor to differentiate between the two types of signals and to process each type appropriately.
In addition to identifying whether a received signal is associated with a target emitter or a reference beacon, the reference correction processor may also need to associate a given reference correction signal with the individual reference beacon that transmitted it, so that its location can be determined.
TDOA/FDOA geolocation systems currently deployed by the U.S. Government differentiate between target emitters and reference beacons and identify the source of a particular reference signal by the fact that the geolocation (or reference correction) processor directly controls the times and frequencies of the reference beacon transmissions. The geolocation processor knows a priori when in time and at which frequency transmissions from a particular reference beacon will arrive at the collectors associated with the system. This precludes the possibility of a reference beacon randomly initiating reference signal transmissions without coordination with the geolocation processor.
In order to control the transmissions of a reference beacon from a geolocation processor, a communications link is required between the two. Such a link is typically provided by a data communications network. Even with such an arrangement, however, the determination that a signal received by a geolocation system is that of a specific reference beacon, or of a reference beacon at all, is not entirely unambiguous.